Presuming you are trying to avoid putting a router in place, technically I can imagine a backwards way to make this work. This would not be something you’d want to do. Youd have to rely on the primary firewall to route the block which would make the secondary dependent on it which makes actual failover not possible.
Most ISPs (at least in the US) will hand off a /29 directly these days and that would be the easiest way to get failover working without having to buy or rent a router
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